1. Field of the Invention
The present invention relates in general to new methods of producing or creating shape memory polymers (SMPs) that are high temperature compatible. The SMP is prepared by subjecting a thermoplastic resin to a high thermal stability radical source under extrusion conditions.
2. Description of the Prior Art
The first materials to be reported as exhibiting a shape memory effect were shape memory alloys (SMAs). Shape memory polymers (SMPs) typically exhibit a higher percentage of strain recovery and shape change than SMAs. Additionally, lower cost and better processability make SMPs viable for implementation into an array of applications.
SMPs are a unique class of polymers that soften and harden quickly and repetitively, on demand. The SMP's ability to transition from a soft to a hard state within a very narrow temperature span is a key physical property that allows processed SMP materials to maintain full structural rigidity up to a specifically designed activation, or glass transition temperature (Tg), and become softened with a slight elevation in temperature above Tg.
By conventional processing, i.e., extruding or injection molding, the polymer is formed into its initial, permanent shape. Afterwards, in a process called programming, the polymer is deformed and fixed into a temporary shape. Upon application of an external stimulus (i.e., heat, light), the polymer recovers its initial permanent shape. This cycle of programming and recovery can be repeated several times, with different temporary shapes in subsequent cycles.
Since the initial discovery of SMPs, many applications have been discovered for these unique materials that allow triggered responses. SMPs have been used in, for example, heat-expanding foam to seal windows, heat-shrinkable tapes, information storage that can allow thermally reversible recording, temperature sensors, smart clothing, and biomedical applications such as actuators, biodegradable sutures actuators, catheters, and smart stents.
Although a number of SMP systems have been developed, there is still a need for a high-temperature compatible SMP extrusion or injection molding processes. Most SMPs to date have a Tg of less than 100° C. The currently available high temperature compatible SMP rely on long cure cycles and expensive starting materials. It is desirable to have a more economical and timely manner to manufacture SMPs that are stable at elevated temperatures, specifically for tooling purposes.